Adriamycin is an antineoplastic compound effective against a number of tumors. A detailed understanding of the mechanism of action of adriamycin has been hindered due to the reversibility of the drug receptor interactions. The objective of this research is to study, by photoaffinity labeling, the interactions of the drug with cellular macromolecules in order to determine the mechanism of action of toxic and therapeutic effects of the drug. Photoaffinity labeling involves irradiation of the non-covalent adriamycin-receptor complex in order to generate a moiety of adriamycin that will react covalently at the same site. Photolytic drug binding in vivo should be accompanied by photolytic enhancement of biological responses. Indeed, there is photoenhancement of several biological properties of adriamycin: killing of L1210 leukemia cells, inhibition of DNA synthesis and inhibition of RNA synthesis. The subcellular distribution of adriamycin is also altered by photolysis. Those subcellular fractions showing an interaction with adriamycin will be fractionated into RNA, DNA, protein and lipids to further localize the drug receptor. Those macromolecules to which adriamycin is bound will be degraded and the monomeric units examined for their interaction with adriamycin. The major value of this research lies in unambiguous identification of drug receptors in the cells. It is hoped that this new approach to the mechanism of action of antineoplastic compounds will allow the determination of the interaction of adriamycin with cellular macromolecules and the relationship between altered structure and function.